The present invention relates to voltage regulators and, more particularly, to an integrated circuit (IC) bandgap voltage reference circuit.
Prior art bandgap voltage reference circuits, which are suitable to be produced in IC form, are well known. Typically, these circuits develop an output voltage having substantially zero temperature coefficient which is obtained by combining two potentials having complimentary temperature coefficients, i.e., one potential having a positive temperature coefficient while the other has a negative temperature coefficient.
In general, the two potentials are produced by using two transistors operated at different current densities as is well understood. By connecting a resistor in series with the emitter of the transistor that is operated at a smaller current density and then coupling the base of this transistor and the other end of the resistor across the base and emitter of the transistor operated at the higher current density produces a delta V.sub.BE voltage across the resistor that has a positive temperature coefficient. This positive temperature coefficient voltage is combined in series with the V.sub.BE of the second transistor which has a negative temperature coefficient in a manner to produce a composite voltage having a very low or zero temperature coefficient. These prior art voltage reference circuits are generally referred to as bandgap voltage references because the composite voltage is nearly equal to the bandgap voltage of silicon semiconductor material, i.e., approximately 1.2 volts.
Most good quality integrated bandgap voltage reference circuits of the type described above require high gain, high quality PNP transistors for sourcing currents to the first and second standard transistors bandgap cell. Typically, the two transistors of the bandgap cell are NPN devices with the first transistor having an emitter area that is ratioed with respect to the emitter area of the second transistor whereby the difference in the current density is established by maintaining the collector currents of the two transistors equal.
These prior art circuits are manufactured in integrated circuit form using contemporary high voltage semiconductor processes such that the required PNP transistors have excellent matched characteristics as well as high output impedances and high forward current gain. Such is not the case in most present day low voltage semiconductor processes. For example, in most, if not all, contemporary low voltage semiconductor processes, the PNP devices cannot be matched to tolerable tolerances and suffer both in their output impedance and forward current gain. Thus, the currents produced by PNP's formed using contemporary low voltage semiconductor processes cannot be matched nor maintained substantially the same from one process to the next or even from one circuit to the next using today's low voltage processes. Thus, practical low voltage bandgap reference circuits cannot be manufactured utilizing present day high speed, low voltage semiconductor processes because of the poor quality of the PNP current source transistors.
Hence, a need exists for a low voltage reference circuit for providing a bandgap reference voltage having excellent temperature performance, power supply rejection and load regulation that does not require well matched, high gain PNP transistors in the integrated circuit.